IMO, the notes in the PDF (and my imagination in translating them) don't seem to clarify anything. Which is why I asked. I'm glad I did, because I like your interpretation.
The area surrounding Silbury is frequently flooded, Pilgrim, and that could account for the alluvial band (natural deposit). But if Silbury was in fact designed as an 'island' then a clay covering, as Squid Tempest suggests, would also make perfect sense.